1. Field of the Invention
The present invention relates to a technique utilizing an electromagnetic wave of a band in the range from about 0.1×1012 Hz to about 100×1012 Hz (a terahertz wave).
2. Description of the Related Art
Terahertz wave spectroscopic techniques in the related art include terahertz time-domain spectroscopy (THz-TDS). It is known that terahertz time-domain spectroscopy is suitable for imaging of samples, and the technique attracting attentions in various technical fields such as industrial, medical, biological, agricultural, and security-related techniques.
In terahertz time-domain spectroscopy, pulse light from an ultra-short laser light source is separated into pumping light and probe light, and the pumping light is collected by a terahertz wave generating section. As a result, a current on the order of sub-pico seconds is generated or electrical polarization occurs at the terahertz wave generating section, and a terahertz wave having an electric field amplitude in proportion to a time-differential of the current is generated. The terahertz wave travels via an optical system to a sample to be measured be transmitted or reflected by the sample, and the wave is collected by a terahertz wave detecting section. When the terahertz wave detecting section is irradiated by the probe light at this time, a carrier is generated, and the carrier is accelerated by the electric field of the terahertz wave to generate a current that is an electric signal in the form of pulses. A temporal waveform of the electric field of the terahertz wave amplitude can be measured by shifting the timing at which the probe light arrives at the terahertz wave detecting section. The temporal waveform can be Fourier-transformed to obtain a transmission or reflection spectrum of the terahertz wave band.
An apparatus utilizing the terahertz time-domain spectroscopy has been proposed as follows. Among terahertz electromagnetic waves 20 guided from a radiation antenna 3 via a semi-spherical lens 43 and a parabolic mirror 4, only a particular polarized electromagnetic wave is transmitted through a polarizer 30 that is a wire grid, and the wave is thereafter made to impinge on a sample 5 obliquely. A terahertz electromagnetic wave 21 obtained as a result of reflection at the sample 5 is guided to a reception antenna 7 via a detector 31 that is a wire grid similar to the polarizer 30 (see Japanese Patent No. 3550381 (Patent Document 1)).